Ever since the acquired immunodeficiency syndrome (AIDS) was first recognized among unrelated homosexual men in San Francisco in 1981, it has been identified as a severe and devastating disease worldwide. AIDS has weighed heavily on all our consciences. We have all watched, with grief in our hearts, the uncompromising human suffering this disease has unleashed on the world. No region of the world has been spared the misery brought about by AIDS and HIV infection, and as of July 1990, a total of 266 098 cases of AIDS had been officially report to the World Health Organization by 156 Member States. In this presentation I will review the main epidemiological features of the disease worldwide and discuss the current strategies for prevention and control, as well as the possible impact HIV/AIDS has had and will have in the 1990s.

AIDS epidemiology

Cases have been reported from all continents; the numbers continue to change as infections with the virus HIV are introduced and spread in the unaffected communities. To date, the largest numbers of reported cases have been from the Americas, accounting for just over 162 000 cases and Africa with 64 000 cases.

The reported cases, however, represent a gross underestimate of the cases that have occurred. The global data on AIDS are biased by wide intercountry and interregional variations in case detection and reporting. Completeness of reporting is thought to vary from 80% in some industrialized countries to less than 10% in some African countries. Overall, therefore, we believe that the 266 000 cases that have been officially reported to WHO represent only 40% of the cases that have occurred worldwide. In addition, because of difficulties in diagnosing AIDS in children, these cases are not reported in many countries.

Magnitude of the epidemic

The use of reported cases of AIDS cannot be relied on to accurately assess the magnitude of the epidemic. It is clear that the cases of AIDS that are occurring now are due to HIV infections which were spread silently and extensively in the late 1970s and early 1980s. Since HIV infection precedes the development of AIDS by many years, an optimal understanding of the patterns of AIDS must be obtained from analysis of both reported cases and surveys of infection with HIV. Three distinct global patterns of HIV infection and AIDS have been described. The explanation for these patterns includes differences in temporal spread of HIV among different populations.

HIV infection throughout the world has continued to be limited to three primary modes of transmission: (1) through sexual intercourse; (2) through infected blood or blood products; and (3) from an HIV-infected woman to her fetus or infant. Because of these modes of transmission, the majority of cases continue to be transmitted by voluntary human behaviours; predominantly sexual intercourse and intravenous drug use. Consequently, HIV infections are not uniformly distributed in any population, but disproportionately affect certain identifiable groups of individuals whose behaviour places them at greater risk of HIV infection. Such individuals therefore are likely to be the major source of propagation of the epidemic.

Global patterns of HIV infection and AIDS

Three global patterns are described. Epidemiologic pattern I is found in North America, Western Europe, Australia, New Zealand, and parts of Latin America. In this pattern, HIV spread extensively in the late 1970s and early 1980s, with most cases being in homosexual or bisexual men and intravenous drug users and only a small percentage of cases being transmitted heterosexually. Recent trends, however, indicate that the incidence of new infections among certain groups of homosexual men has slowed down. The full significance of such data is difficult to assess, as there are many groups and subgroups of populations at risk who have either not received much public attention or have not been researched as extensively. Furthermore, in most places, including the United States of America, the size of the populations at risk of infection remains incompletely established. In addition, HIV infection continues to spread in the most socially and economically vulnerable segments of society.

Epidemiologic pattern II is seen in sub-Saharan Africa, Latin America, and the Caribbean. Within areas currently classified in this pattern, transmission continues to be predominantly heterosexual, and the incidence of HIV infection continues to rise. Today, HIV prevalence among women attending antenatal clinics in many urban areas in sub-Saharan Africa ranges from 10% to 30%, while HIV seroprevalence among adults may be as high as 12% in the highly populated areas. Rural areas of many pattern II countries still have low seroprevalences. However, there is still concern that the greater population in these rural areas includes a large pool of individuals whose practices and behaviour place them at risk of HIV infection.

Pattern III areas are areas where few cases of HIV infections have occurred. The HIV pandemic has, however, continued it geographical expansion and has reached these areas and regions previously only slightly affected. Pattern III areas are characterized by the recent onset of the HIV/AIDS pandemic, in the late 1980s. Most countries that exhibit this pattern have not yet shown the predominant modes of transmission.

This, however, changes rapidly; for example, in Bangkok, Thailand, extensive spread of HIV infection among intravenous drug users since early 1988 has been documented, and HIV prevalence estimates rose from about 1% in late 1987 to over 40% in early 1989. A similar rapid rise appears to have occurred in some groups of intravenous drug users in Burma.

HIV prevalence rates

Up to the end of June, WHO estimated that worldwide, 6-8 million persons had become infected with HIV. Recently, WHO revised the global estimates of HIV infection to 8-10 million people around the world. These new figures reflect the continued worsening of the epidemic of HIV/AIDS in developing countries, especially in sub-Saharan Africa and Asia. The new estimates indicate the following trends.

Sub-Saharan Africa: WHO estimates of persons infected in sub-Saharan Africa have increased from 3 million to about 5 million. In 1987, most HIV-infected people were in urban populations. Now, however, extensive spread is being documented in rural areas. It is estimated now that about one in every 40 adult men and women is infected with HIV.

Asia: Serological data for 1988 and 1989 for South-east Asian countries, including Thailand and India, indicate marked increases in HIV infections among intravenous drug users and female prostitutes. HIV was introduced into Asia only in the early- to mid-1980s, and up to the end of the 1980s the numbers of both AIDS and HIV infections were low. Recent data indicated that the total number of HIV-infected persons in Asia has risen from virtually nil two years ago to an estimated current total of at least 500 000, a much more rapid increase than projected even a year ago.

What does the future hold for AIDS?

During the first decade of the pandemic, AIDS/HIV has caused an estimated 500 000 cases in women and children, most of which have so far been unrecognized. During the 1990s, the pandemic will kill an additional 3 million or more women and children throughout the world, and it is estimated that up to 1 million uninfected children will have been orphaned because their HIV-infected mothers and fathers will have died from AIDS.

For the 1990s, AIDS may take other important turns - there may be growing complacency among policy-makers and an eventual decrease in funds for control and prevention activities.

However, more cases of HIV infection will continue to occur worldwide, and there will be a continued increase in sick persons to be taken care of by already overstretched health care systems. Numerous challenges still exist in prevention and control of AIDS worldwide:

Firstly, the commonest mode of transmission, sexual transmission. It is well recognized that to prevent sexual transmission requires: (1) initiatives from persons at risk, and (2) identification of factors that may increase the risk of transmission, such as other sexually transmitted diseases.

Secondly, although blood and blood product transmission is relatively easy to prevent, there still exists marked potential for transmission, especially in any situation where there is exchange of blood, as seen in intravenous drug Users and hospital settings where reuse of needles and syringes is common practice. This has been shown in Romania and the Soviet Union.

Thirdly, transmission from infected women to their infants is also difficult to prevent, and the problem of transmission will continue to become bigger. Potential interventions however include: (1) safer sexual behaviour/family planning; (2) voluntary antenatal screening; (3) possibly pharmaceuticals; and (4) pregnancy termination.

Conclusion

The challenge that still remains, however, in AIDS prevention and control is to assure an appropriate balance of (1) prevention services, and (2) control of the impact of AIDS on HIV-infected persons and their families and friends.

There is a difficult task ahead. AIDS is revealing the many inadequacies in our societies today, and we must continue to fight and avoid complacency as the epidemiology of the disease and future trends only indicate a worsening problem. Effective prevention and control efforts will require a concerted multidisciplinary approach, which should include all sectors of the health care system.